1. Field of the Invention
The present invention generally relates to the production of synthesis gas. More particularly, the invention relates to supported catalysts and processes for the catalytic partial oxidation of light hydrocarbons (e.g., methane or natural gas) to produce a mixture of primarily carbon monoxide and hydrogen (synthesis gas). The invention also relates to methods of preparing a catalyst or catalyst support material having properties that provide low-temperature light-off of the direct catalytic partial oxidation reaction and enhance the production of synthesis gas.
2. Description of Related Art
Large quantities of methane, the main component of natural gas, are available in many areas of the world, and natural gas is predicted to outlast oil reserves by a significant margin. However, most natural gas is situated in areas that are geographically remote from population and industrial centers. The costs of compression, transportation, and storage make its use economically unattractive.
To improve the economics of natural gas use, much research has focused on methane as a starting material for the production of higher hydrocarbons and hydrocarbon liquids. The conversion of methane to hydrocarbons is typically carried out in two steps. In the first step, methane is reformed with water to produce carbon monoxide and hydrogen (i.e., synthesis gas or syngas). In a second step, the syngas intermediate is converted to higher hydrocarbon products by processes such as the Fischer-Tropsch Synthesis. For example, fuels with boiling points in the middle distillate range, such as kerosene and diesel fuel, and hydrocarbon waxes may be produced from the synthesis gas.
Current industrial use of methane as a chemical feedstock proceeds by the initial conversion of methane to carbon monoxide and hydrogen by either steam reforming or dry reforming. Steam reforming currently is the major process used commercially for the conversion of methane to synthesis gas, the reaction proceeding according to Equation 1.CH4+H2O⇄CO+3H2  (1)
Although steam reforming has been practiced for over five decades, efforts to improve the energy efficiency and reduce the capital investment required for this technology continue. The steam reforming reaction is endothermic (about 49 kcal/mol), requiring the expenditure of large amounts of fuel to produce the necessary beat for the industrial scale process. Another drawback of steam reforming is that for many industrial applications, the 3:1 ratio of H2:CO products is problematic, and the typically large steam reforming plants are not practical to set up at remote sites of natural gas formations.
The catalytic partial oxidation (“CPOX”) of hydrocarbons, e.g., methane or natural gas, to syngas has also been described in the literature. In catalytic partial oxidation, natural gas is mixed with air, oxygen-enriched air, or oxygen, and introduced to a catalyst at elevated temperature and pressure. The partial or direct oxidation of methane yields a syngas mixture with a H2:CO ratio of 2:1, as shown in Equation 2.CH4+½O2→CO+2H2  (2)
This ratio is more useful than the H2:CO ratio from steam reforming for the downstream conversion of the syngas to chemicals such as methanol or to fuels. The CPOX reaction is exothermic (−8.5 kcal/mol), in contrast to the strongly endothermic steam reforming reaction. Furthermore, oxidation reactions are typically much faster than reforming reactions. This allows the use of much smaller reactors for catalytic partial oxidation processes than is possible in a conventional steam reforming process.
While its use is currently limited as an industrial process, CPOX of methane has recently attracted much attention due to its inherent advantages, such as the fact that due to the significant heat that is released during the process, there is no requirement for the continuous input of heat in order to maintain the reaction, in contrast to steam reforming processes. An attempt to overcome some of the disadvantages and costs typical of steam reforming by production of synthesis gas via the catalytic partial oxidation of methane is described in European Patent No. 303,438. According to that method, certain high surface area monoliths coated with metals or metal oxides that are active as oxidation catalysts, e.g., Pd, Pt, Rh, Ir, Os, Ru, Ni, Cr, Co, Ce, La, and mixtures thereof, are employed as catalysts. Other suggested coating metals are noble metals and metals of groups IA, IIA, III, IV, VB, VIB, or VIIB of the periodic table of the elements.
Other methane oxidation reactions include the highly exothermic combustion (−192 kcaL/mol) and partial combustion (−124 kcal/mol) reactions, Equations 3 and 4, respectively.CH4+2 O2→CO2+2H2O  (3)CH4+3/2 O2→CO+2H2O  (4)
U.S. Pat. No. 5,149,464 describes a method for selectively converting methane to syngas at 650–950° C. by contacting a methane/oxygen mixture with a solid catalyst which is a d-block transition metal on a refractory support, an oxide of a d-block transition metal, or a compound of the formula MxM′yOz wherein M′ is a d-block transition metal and M is Mg, B, Al, Ga, Si, Ti, Zr, Hf or a lanthanide.
U.S. Pat. No. 5,500,149 describes the combination of dry reforming and partial oxidation of methane, in the presence of added CO2 to enhance the selectivity and degree of conversion to synthesis gas. The catalyst is a d-block transition metal or oxide such as a group VIII metal on a metal oxide support such as alumina, is made by precipitating the metal oxides, or precursors thereof such as carbonates or nitrates or any thermally decomposable salts, onto a refractory solid which may itself be massive or particulate; or one metal oxide or precursor may be precipitated onto the other. Preferred catalyst precursors are those having the catalytic metal highly dispersed-on an inert metal oxide support and in a form readily reducible to the elemental state.
For successful commercial scale operation a catalytic partial oxidation process must be able to achieve and sustain a high conversion of the methane feedstock at high gas hourly space velocities, with high selectivity for the desired H2 and CO products. Moreover, such high conversion and selectivity levels must be achieved without detrimental effects to the catalyst, such as the formation of carbon deposits (“coke”) on the catalyst, which severely reduces catalyst performance. The choice of catalyst composition and the manner in which the catalyst is made are important factors in determining whether a catalyst will have sufficient physical and chemical stability to operate satisfactorily for extended periods of time on stream at moderate to high temperatures and will avoid high pressure drop in a syngas production operation.
In most of the existing syngas production processes it is difficult to select a catalyst that will be economical for large scale industrial use, yet will provide the desired level of activity and selectivity for CO and H2 and demonstrate long on-stream life. Today, metal oxide supported noble metal catalysts or mixed metal oxide catalysts are most commonly used for the selective oxidation of hydrocarbons and for catalytic combustion processes. Various techniques are employed to prepare the catalysts, including impregnation, washcoating, xerogel, aerogel or sol gel formation, spray drying and spray roasting. Monolith supported catalysts having pores or longitudinal channels or passageways are commonly used. Such catalyst forming techniques and configurations are well described in the literature, for example, in Structured Catalysts and Reactors, A. Cybulski and J. A. Moulijn (Eds.), Marcel Dekker, Inc., 1998, p. 599–615 (Ch. 21, X. Xu and J. A. Moulijn, “Transformation of a Structured Carrier into Structured Catalyst”).
U.S. Pat. No. 5,510,056 discloses a ceramic foam supported Ru, Rh, Pd, Os, Ir or Pt catalyst having a specified tortuosity and number of interstitial pores that is said to allow operation at high gas space velocity. The catalyst is prepared by depositing the metal on a carrier using an impregnation technique, which typically comprises contacting the carrier material with a solution of a compound of the catalytically active metal, followed by drying and calcining the resulting material. The catalyst is employed for the catalytic partial oxidation of a hydrocarbon feedstock.
U.S. Pat. No. 5,648,582 discloses a rhodium or platinum catalyst prepared by washcoating an alumina foam monolith having an open, cellular, sponge-like structure. The catalyst is used for the catalytic partial oxidation of methane at space velocities of 120,000 h.−1 to 12,000,000 h−1 
More recently, particulate syngas catalysts have been found to offer certain advantages over monolithic catalysts. For example, Hohn and Schmidt (Applied Catalysis A: General 211:53–68 (2001)) compare monolith and particulate (i.e., sphere) beds in a catalytic partial oxidation process, and shows that a non-porous alumina support gave superior results for the production of synthesis gas, even at space velocities of 1.8×106 h−1, compared to a comparable alumina monolith support. The internal surface area of the support material is apparently unimportant, as the pre-sintered and unsintered alumina spheres (surface area about 10 m2/g and about 200 m2/g, respectively) gave the same results after loading with the same amount of Rh. A major advantage of those particle beds is said to be better heat transfer than the corresponding monolithic catalyst.
U.S. Patent Application No. 2001/0027258 A1 describes a catalytic partial oxidation process that includes contacting a C1–C4 hydrocarbon and oxygen with a bed of particulate, supported Group VIII metal catalyst. The support has a surface to volume ratio of about 15–230 cm−1 and the preferred catalyst particle size range is about 200–2000 microns in diameter. The preferred support particles generally have a low total surface area, e.g., <20 m2/gm, and microporosity is not important to the process.
PCT Publication No. WO 02/20395 (Conoco Inc.) describes certain rhodium-based catalysts that are active for catalyzing the net partial oxidation of methane to CO and H2. A preferred catalyst comprises highly dispersed, high surface area rhodium on a granular zirconia support with an intermediate coating of a lanthanide and/or lanthanide oxide. The catalyst is thermally conditioned during its preparation.
In order to initiate a CPOX process, it is typically necessary to preheat the catalyst to a temperature at which ignition (i.e., initiation of the CPOX reaction) occurs. This can be problematic, however, because CPOX syngas reactors are small, and providing for an additional ignition source for catalyst heating can complicate the process and significantly add to the size and cost of the syngas reactor system. For example, U.S. Pat. No. 6,329,434 describes an alternative to conventional light-off procedures and equipment. It is pointed out that conventional methods, such as use of a preheating torch or burner, are not practical for catalytic partial oxidation processes, and that too rapid heating of the catalyst bed can destroy the catalyst due to thermal stresses. An O2 and H2 feed, with a diluent, is employed to ignite the catalyst bed and control heat up. Suggested diluents include nitrogen, helium, argon, steam, methane, CO, CO2, ethane, propane, butane, alcohols and olefins.
A technique in common use today for initiating CPOX processes is pre-heating the hydrocarbon feed up to 500° C., or more, before contacting the catalyst. A drawback of pre-heating the hydrocarbon is the increased hazard it presents when hot hydrocarbon is combined with an O2-containing stream. There is also a risk of hydrocracking during preheating of the hydrocarbon feed. Moreover, including a preheat facility increases the capital cost of the syngas production unit. In some CPOX processes, the hydrocarbon feed is briefly spiked or supplemented with an “ignition agent,” which is typically a partially oxidizable gas that is more readily oxidizable than methane, natural gas or a mixture of C1–C5 gases at a given temperature. For example, up to about 50 vol % propane in the hydrocarbon feed might be needed in order to initiate the CPOX reaction, keeping the pre-heat temperature below 500° C. PCT Publication No. WO 99/35082 describes starting a CPOX reactor from ambient temperature by using a mixture of light hydrocarbons or ammonia and air preheated at 200° C., and then introducing the gaseous mixture to the catalyst at an appropriate temperature at which combustion will occur.
Use of a supplemental ignition agent such as propane complicates the syngas production procedure, and the associated refrigeration equipment adds to the cost of the system. There are additional safety considerations, and detrimental effects on the efficiency of the syngas production process are possible due to coke deposition. Although significant advances have been made in the development of catalysts and processes for producing synthesis gas, in order for catalytic partial oxidation processes to be commercially feasible there continues to be a need for more efficient and economical processes and catalysts. At the present time, there is no commercially practical CPOX reaction system for the manufacture of syngas. The syngas production process needs to be easier to practice, not dependent upon additional ignition sources, and capable of lighting off at low temperatures. Ideal syngas catalysts would also be physically and chemically stable on stream, resist coking, and also retain a high level of conversion activity and selectivity to carbon monoxide and hydrogen under the conditions of high gas space velocity and elevated pressure that are needed for achieving high space time syngas yield.